1. Field of Invention
The present invention relates to radar detection and automatic tracking; and more particularly, to a surveillance pulse Doppler radar system and method using scan to scan correlation for the detection and tracking of maneuvering and non-maneuvering targets.
2. Background of Invention
A pulsed radar system that extracts the Doppler frequency shift of return radar pulses for the purpose of detecting moving targets, and determining their radial velocity, is referred to as a pulse Doppler radar. A pulse Doppler radar transmits, repetitively, modulated pulses for a certain time duration, which is referred to as the dwell time of the radar. The time period between each pulse or modulation is the interpulse period, and determines the pulse repetition frequency (PRF). It is common practice to vary the interpulse periods, thus providing several pulse repetition frequencies or PRF's. Such radar systems typically have a relatively high PRF in order to prevent ambiguities in the Doppler frequency shift. However, in preventing ambiguity of the Doppler frequency shift, such relatively high PRF's result in range ambiguities.
Pulse Doppler radar systems are presently used in airborne surveillance systems, which have a highly directional antenna that scans mechanically in azimuth at approximately six revolutions per minute; and in some modes, simultaneously scans electronically in elevation.
In typical prior art systems, it is customary to reject main beam clutter by filtering the output of an A/D converter for rejecting signals which are Doppler shifted by an amount corresponding to such main beam clutter. Hence, the main beam clutter, which has a predictable Doppler shift, is rejected by a band pass (notch) filter. The time domain signals are then transformed to the Doppler domain by the use of an FFT (Fast Fourier transform). The output of the filter bank is fed into a constant false alarm rate (CFAR) threshold circuit. This threshold, in prior art systems, was required to be sufficiently high to reject most of the undesired signals. Moreover, the CFAR is relatively ineffective against clutter signals. Each detected range gate filter out of the CFAR is Doppler centroided; and is then subjected to post-detection processing, wherein the filters are unfolded and normalized to a single PRF, to determine unambiguous radial velocity with multiple PRF's. The detected signals are then correlated in Doppler and range using multiple PRF's. In current airborne surveillance radars, a detection on three (3 of 3) successive PRF's is required before a correlation is declared. Correlations of 2 out of 3, or 2 out of 2 return signals are not considered to be correlations. Although the results of using the most severe post-detection processing (3 of 3) is effective in preventing false alarms and providing excellent performance on today's airborne threat, it is insufficient to provide an acceptable blip/scan ratio for future threats.
The low blip/scan ratio results in an inordinate use of the "coast" feature in radar tracking; that is, the feature where the predicted position of a target is extrapolated upon the failure to detect a target on one or more scans. In such situations, a maneuvering target is difficult to track with a ten second data interval.